The project seeks to provide information on the existence and anatomical definition and extent of steroid hormone target cells in the brain of phylogenetic representatives of different vertebrate classes and in neonatal and fetal rats, determining also the ontogenetic appearance of steroid "receptors". In depth studies with 3H estradiol-17-beta in the rat are aimed at the quantitative assessment of differences in estrogen-"receptor" concentration related to different postnatal developmental states, male and female sex, and organizational and activational hormone induced modifications. Characterization of estrogen target cells will be attempted by using recently developed combinations of catecholamine fluorescence or immunohistochemical staining with anti-gonadotropin with the auto-radiographic 3H estradiol localization as well as by using electron miroscopic autoradiography. Apparent differences in "receptor" affinity to steroid hormones within brain regions and between brain, pituitary and other targets will be measured and evaluated quantitatively. Pharmacological antagonists will be tested. Selective inhibition of "receptor" binding could be of therapeutic potential.